1. Field of the Invention
This invention relates to a process for preparing an aqueous solution of acrylamide. More specifically, the invention relates to an improvement in a process which comprises catalytically hydrating acrylonitrile in the presence of a metal-containing catalyst to form a solution of acrylamide, acrylonitrile and water, and increasing the concentration of acrylamide in the aqueous solution by removing the acrylonitrile and water.
2. Description of the Prior Art
Acrylamide is useful as a starting material for the production of polymers. For use as such a starting material, acrylamide must have the lowest possible acrylonitrile content, and the concentration of residual acrylonitrile should normally be less than about 0.1% by weight.
Previous practice e.g., as disclosed in U.S. Pat. Nos. 3,923,741 and 3,941,837, has been to use a distillation tower having a rectifying section and a heat evaporating section in order to obtain an aqueous solution of acrylamide from a mixed solution of acrylamide, acrylonitrile and water.
For heat evaporation, a stirred tank-type evaporation still, a forced circulation-type evaporation still or a horizontal cocurrent centrifugal evaporator has previously been used. When such a heat evaporator is used, it is difficult to reduce the concentration of acrylonitrile remaining in the solution to about 0.1% or less. It has been necessary, therefore, to reduce the acrylonitrile concentration by rectification.
A packed tower or a plate tower is used in the rectification section, and during rectification, the solution adheres to the inner wall of the rectifying tower, the inner wall of the packing, the undersurface of the trays, etc. Thus, these portions are not refreshed, and a polymerization of the adhering acrylamide may occur. The acrylamide polymer is water-soluble, and the inclusion of even a small amount of this polymer in the acrylamide aqueous solution decreases the quality. Furthermore, in order to prevent the polymerization of acrylamide during heat concentration, the concentrating operation must be performed at lower pressures and temperatures. Since a pressure drop occurs at the rectification section, the pressure at the condenser should be further reduced, making it difficult to condense the acrylonitrile.